1. Field of the Invention
The present invention relates to oil seals for establishing a seal between relatively movable shaft and housing. More particularly, the present invention relates to lip-type oil seals having hydrodynamic pumping means, which is used in an automobile or in a general industrial machinery.
2. Description of the Prior Art
Conventionally, as shown in FIG. 15, there has been known an oil seal having helical pumping ribs 52 at around a tip portion of a seal lip 51 to improve seal functions. The pumping ribs provide oil seals with a pumping function to pump fluid to be sealed toward fluid side. This type of oil seals has a sharp edge 53 at a tip of the seal lip 51, and the helical pumping ribs 52 starts from the sharp edge 53.
The sharp edge 53, however, tends to be abraded comparatively rapidly by sliding on a member to be sealed such as a shaft (not shown in FIG. 15).
Therefor, in these conventional oil seals mentioned above, the tip of the helical pumping ribs 52 tend to be comparatively rapidly disappeared with comparatively rapidly being abraded at the sharp edge 53, causing changes of contact conditions of the helical pumping ribs 52 with the member to be sealed.
Accordingly, there have been a defect of comparatively rapidly decrease in pumping function of the helical pumping ribs 52, by comparatively rapidly abrasion phenomenon mentioned above.
Japan unexamined laid open patent 7-301339 discloses an oil seal having, in the seal lip 101, a third frustoconical surface (so called a "flat" surface) 106 between a frustoconical air-side surface 103 and a frustoconical oil-side surface 102 (as is shown in FIG. 16) to avoid such a defect as described above. There are an edge line 104 and 107, which is a juncture line between the flat surface 106 and the frustoconical oil-side surface 102, and between the flat surface 106 and the frustoconical air-side surface 103, respectively as shown in FIG. 16. As is shown in FIG. 16, the flat surface has a width W in actual size, which is a width in the direction of a central axis line 0 of the oil seal. The oil seal also has angles a, .beta., and .gamma. as is shown in FIG. 16; the .alpha., .beta. and .gamma. is an angle between the central axis of the oil seal and the frustoconical oil-side surface 102, the frustoconical air-side surface 103, and the so-called flat surface 106, respectively. The patent of 7-301339 also discloses uniform helical grooves providing the seal lip with a pumping function pumping oil (or fluid, not shown in FIG. 16) back from the air-side to the oil-side. In this regard, the terms "oil-side" and "air-side" refer, respectively, to being situated closer to the oil-side A and the air-side B indicated in FIG. 16.
The oil seal also has a garter spring 105 placed in a groove formed on an outer periphery of the seal lip 101, and a rigid annular casing 108 which supports the seal lip 101 in the lower part of the oil seal, as is shown in FIG. 16.
The 7-301339 patent, however, has such a wider projection area which is placed between a groove and an another adjacent groove, that a deformation of the groove is not so large at the time when the third frustoconical surface is fitted on a member (or shaft, not shown in FIG. 16) to be sealed. This type of oil seals show sometimes some disadvantages such as some oil leakage from the oil-side A to the air-side B through a portion between the seal lip and the member (shaft) to be sealed, at the time of the state that there is no relative rotational movement between the seal lip and the member (shaft) to be sealed, because of the groove being usually little deformed to cause some apertures between the seal lip and the shaft.